1. Field of the Invention
The present invention relates to a side light type spread illuminating apparatus, and more particularly to a spread illuminating apparatus for use as a lighting means for a liquid crystal display device.
2. Description of the Related Art
A side light type spread illuminating apparatus, in which a primary light source is disposed at a side surface of a light conductor plate, is predominantly used as a lighting means for a liquid crystal display (LCD) device used in a mobile telephone, and like devices. Conventionally, the primary light source has been constituted by a cold cathode lamp. Currently, a point light source, such as a white light emitting diode (LED), is heavily used, which is easier to handle, enables easier downsizing and is more resistant to impact shock than the cold cathode lamp.
Such a spread illuminating apparatus using a point light source is expanding its application field, and the application is now directed not only to a small LCD device for use in a mobile telephone but also to a relatively large LCD device for use in, for example, a car navigation system. In order to sufficiently illuminate a large display area, various approaches have been attempted to efficiently utilize light emitted from the primary light source and also to increase the amount of light emitted.
One of such approaches is disclosed (refer to, for example, Japanese Patent Application Laid-Open No. 2003-215546: Paragraph [0027] and FIG. 2 therein), in which an LED is disposed tightly close to a light conductor plate so that light emitted from the LED can be efficiently introduced into the light conductor plate. FIG. 4 shows a conventional spread illuminating apparatus incorporating the aforementioned approach, in which a light inlet surface LP of a light conductor plate GLB makes contact with the light emitting portions of LEDs 1 and 2, and inward curvatures PJ1 and PJ2 are formed at one side LW2 of a frame-like molded case MLD so as to protrude toward the LEDs 1 and 2 thereby resiliently generating forces (F) to press the light conductor plate GLB against the LEDs 1 and 2, which ensures a close contact between the LEDs 1 and 2 and the light inlet surface LP of the light conductor plate GLB thus allowing lights emitted from the LEDs 1 and 2 to be effectively introduced into the light conductor plate GLB.
Another approach is to increase the amount of light emitted from a primary light source by increasing the number of LEDs disposed at a side of a light conductor plate, or by increasing the current applied to an LED so as to increase the light amount per LED. This approach, however, causes an increase in heat generation at the primary light source thus raising the ambient temperature.
Generally, the luminous efficiency of an LED is lowered in proportion to an increase in temperature, and also in the case of the spread illuminating apparatus of FIG. 4 in which the LED and the light inlet surface of the light conductor plate are in contact with each other, heat generated at the LED is transmitted directly to the light inlet surface of the light conductor plate thus increasingly allowing the light inlet surface to suffer damages, such as thermal deformation.
Further, it is known that a side light type spread illuminating apparatus which has a plurality of LEDs disposed at a light inlet surface of a light conductor plate exhibits uneven brightness at a portion of the light conductor plate located close to the light inlet surface, such that there are bright areas positioned in front of the LEDs and dark areas each positioned between two adjacent LEDs. In order to overcome the uneven brightness problem, the side surface (light inlet surface) of the light conductor plate is, for example, provided with a prism array. If a prism array is formed at the light inlet surface LP of the light conductor plate GLB of the spread illuminating apparatus shown in FIG. 4, the heats from the LEDs 1 and 2 and the loads from the pressing forces (F) are concentrated at the apexes of prisms making contact with the LEDs 1 and 2, and consequently the aforementioned thermal deformation is caused more easily.
To deal with the heat generation problem with the LED, the LED and the light conductor plate are housed in a metallic chassis such that the LED makes contact with the chassis for enhancing the radiation performance (refer to, for example, Japanese Patent Application Laid-Open No. 2004-186004: Paragraphs [0035] to [0037] and FIG. 2 therein).
However, in the case of the spread illuminating apparatus of FIG. 4 in which the LED is disposed in contact with the light inlet surface of the light conductor plate, it is difficult to completely eliminate the heat deformation problem only by arranging the LED to make contact with the metallic chassis for enhancing the radiation performance as described above. Thus, as for the prevention of the thermal deformation, it is preferable for the LED not to make contact with the light conductor plate. On the other hand, when the LED is disposed at the light inlet surface of the light conductor plate with a certain air gap provided therebetween, the optical coupling efficiency between the LED and the light conductor plate is deteriorated thus lowering the brightness of the spread illuminating apparatus, and also the difficulty in precisely controlling the air gap distance causes variation in brightness and visual quality.